Exhaust emission control device

ABSTRACT

Even in a vehicle with travel pattern of continuing operational status with low exhaust temperature, a satisfactory NO x  reduction effect can be attained even at exhaust temperature lower than that required conventionally therefor. 
     In an exhaust emission control device with selective reduction catalyst  10  incorporated in an exhaust pipe  9,  ammonia being added upstream of the catalyst  10  for reduction and purification of NO x , the device comprises an ammonia generator  12  with a vessel  15  for holding urea water  23   a  and with an electrode  16  for generation of ammonia  13   a,    13   b  through action of plasma on the urea water  23   a  in the vessel, the ammonia  13   a,    13   b  generated in the generator  12  being fed upstream of the catalyst  10.

TECHNICAL FIELD

The present invention relates to an exhaust emission control deviceapplied to an engine such as diesel engine.

BACKGROUND ART

Conventionally some diesel engines have been provided with selectivereduction catalyst incorporated in an exhaust pipe through which exhaustgas flows, said catalyst having a characteristic of selectively reactingNO_(x) with a reducing agent even in the presence of oxygen. A requiredamount of reducing agent is added upstream of the selective reductioncatalyst and is reacted with NO_(x) (nitrogen oxides) in exhaust gas onthe catalyst to thereby reduce a concentration of the discharged NO_(x).

Meanwhile, it is well known in a field of industrial flue gasdenitration in a plant or the like that ammonia (NH₃) has effectivenessas a reducing agent for reduction and purification of NO_(x). However,in a field of automobile, guaranteed safety is hard to obtain withrespect to travel with ammonia itself being loaded, so that researcheshave been made nowadays on use of nontoxic urea water as reducing agent(see, for example, Reference 1).

More specifically, when added to the exhaust gas upstream of theselective reduction catalyst, the urea water is pyrolytically decomposedby heat of the exhaust gas into ammonia and carbon dioxide according tothe following equation, and NO_(x) in the exhaust gas on the catalyst issatisfactorily reduced and purified by the ammonia generated.

(NH₂)₂CO+H₂O→2NH₃+CO₂

-   [Reference 1] JP 2002-161732A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

It has been experimentally ascertained that, by adding ammonia toselective reduction catalyst in such kind of exhaust emission controldevice, NO_(x) reduction effect will be obtained, provided that exhausttemperature exceeds about 140° C.; however, pyrolytical decomposition ofurea water into ammonia and carbon dioxide requires exhaust temperatureof at least about 170-180° C. Thus, if an operating status with exhausttemperature of lower than about 200° C. continues (generally speaking,low-load operational areas are areas with low exhaust temperature),there is a problem that NO_(x) reduction ratio is hardly to enhancesince decomposition of urea water into ammonia does not proceed well.For example, in a vehicle such as city shuttle-bus with travel patternof almost always traveling on congested roads, an operation with morethan required exhaust temperature does not continue for a long time, andoperational transitions occur with no chance of NO_(x) reduction ratiobeing enhanced, failing in obtaining satisfactory NO_(x) reductioneffect.

The invention was made in view of the above and has its object toprovide an exhaust emission control device which can obtain satisfactoryNO_(x) reduction effect even at exhaust temperature lower than thatrequired conventionally therefor and even in a vehicle with travelpattern of continuing operational status with low exhaust temperature,which can effectively generate ammonia from urea water and which canenhance controllability in adding ammonia to the exhaust gas.

Means or Measures for Solving the Problems

The invention is directed to an exhaust emission control device withselective reduction catalyst incorporated in an exhaust pipe, ammoniabeing added upstream of the catalyst so as to reduce and purify NO_(x),said exhaust emission control device comprising an ammonia generatorwith a vessel for holding urea water and with an electrode forgeneration of ammonia through action of plasma on the urea water in thevessel, the ammonia generated in the ammonia generator being fedupstream of the catalyst.

According to the above means, the ammonia generated through action ofthe plasma on the urea water in the ammonia generator is fed upstream ofthe selective reduction catalyst, so that a required amount of ammoniacan be surely added to the exhaust gas even in an operational statuswith low exhaust temperature to thereby be effectively reacted withNO_(x) in the exhaust gas on the selective reduction catalyst; as aresult, NO_(x) in the exhaust gas is satisfactorily reduced and purifiedeven at exhaust temperature lower than that required conventionallytherefor. Generation of ammonia can be easily and rapidly adjusted sincethe ammonia is generated through action of plasma on the urea water; andresponse in feeding the ammonia can be enhanced since the generatedammonia is added to the exhaust gas.

It is preferable in the exhaust emission control device that dielectricpellets are charged in the urea water in the vessel. Such charging ofthe dielectric pellets in the urea water brings about generation ofplasma on surfaces of the pellets, thereby further effectively enhancingthe action of generating ammonia from the urea water.

In the exhaust emission control device, ammonia gas may be taken outfrom the ammonia generator. Addition of such ammonia gas to the exhaustgas causes no trouble of lowering the exhaust temperature, so thatNO_(x) reduction effect of the selective reduction catalyst in anoperational status with low exhaust temperature can be further enhanced.

In the exhaust emission control device, ammonia water may be taken outfrom the ammonia generator. Addition of such ammonia water to theexhaust gas substantially causes no trouble of lowering the exhausttemperature, though subtle heat may be taken upon evaporation of thewater. Thus, NO_(x) reduction effect of the selective reduction catalystin an operational status with low exhaust temperature can be highlymaintained.

In the exhaust emission control device, a pH meter may be arranged whichdetects concentration of ammonia taken out from the vessel and acontroller may be arranged which outputs a command on amount of ammoniato be fed upstream of the selective reduction catalyst on the basis ofdetected value from the pH meter, whereby actual amount of ammonia to befed to the exhaust gas can be controlled with high response.

EFFECTS OF THE INVENTION

The above-mentioned exhaust emission control device of the invention haseffects and advantages. Ammonia is effectively generated through actionof plasma on urea water in an ammonia generator and is fed upstream ofthe selective reduction catalyst so that, unlike the conventional supplyof urea water, a required amount of ammonia can be surely added toexhaust gas without lowering in temperature of the exhaust gas; thuseven in an operational status with low exhaust temperature, NO_(x) canbe effectively reduced by the selective reduction catalyst. Because ofammonia being generated through action of the plasma on the urea water,the generation of the ammonia can be easily and rapidly adjusted;because of the generated ammonia being added to the exhaust gas,response in feeding the ammonia to the exhaust gas can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic overall diagram showing an exhaust pipe pathway of anengine to which an exhaust emission control device of the invention isapplied.

FIG. 2 A schematic diagram showing an embodiment to take out ammonia gasfrom the ammonia generator shown in FIG. 1.

FIG. 3 A perspective view showing a case where a plurality of vesselseach similar to that of FIG. 2 are provided in module structure.

FIG. 4 A schematic diagram of an embodiment to take out ammonia waterfrom the ammonia generator of FIG. 2.

FIG. 5 A schematic diagram of an embodiment to take out ammonia gas froman ammonia generator different from that shown in FIG. 2.

FIG. 6 A schematic diagram of an embodiment to take out ammonia waterfrom the ammonia generator of FIG. 5.

FIG. 7 A graph showing relationship between exhaust temperature andNO_(x) reduction ratio.

EXPLANATION OF THE REFERENCE NUMERALS

-   9 exhaust pipe-   10 selective reduction catalyst-   12 ammonia generator-   13 ammonia-   13 a ammonia gas (ammonia)-   13 b ammonia water (ammonia)-   15 vessel-   16 electrode-   17 casing-   19 a dielectric pellet-   23 a urea water-   32 controller-   37 pH meter-   38 detected pH value-   39 ammonia feed command-   40 vessel-   41 electrode plate

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be described in conjunction withdrawings.

FIG. 1 is a schematic overall diagram showing an exhaust pipe pathway ofan engine to which an exhaust emission control device of the inventionis applied. In FIG. 1, reference numeral 1 designates an engine such asdiesel engine, the engine 1 illustrated having a turbocharger 2 with acompressor 2 a to which air 4 from an air cleaner 3 is fed through anintake pipe 5. The air 4 thus pressurized in the compressor 2 a isfurther fed to an intercooler 6 where it is cooled. The cooled air 4from the intercooler 6 is guided to an intake manifold (not shown) fromwhich it is guided to respective cylinders of the engine 1.

Exhaust gas discharged from the respective cylinders of the engine 1 isfed via an exhaust manifold 8 to a turbine 2 b of the turbocharger 2.The exhaust gas 7 thus having driven the turbine 2 b is discharged viaan exhaust pipe 9 to outside of the vehicle.

Incorporated in the exhaust pipe 9 through which the exhaust gas 7 flowsis a selective reduction catalyst 10 encased by a casing 11. Theselective reduction catalyst 10 is in the form of a flow-through typehoneycomb structure and has a feature capable of selectively reactingNO_(x) with ammonia even in the presence of oxygen.

In the above construction, the exhaust pipe 9 is provided with a spraynozzle 14 upstream of the casing 11, said nozzle injecting ammonia 13generated in an ammonia generator 12 to add the same to the exhaust gas7.

FIG. 2 shows an embodiment in which ammonia gas 13 a is taken out fromthe ammonia generator 12 of FIG. 1. In the ammonia generator 12, asammonia 13, the ammonia gas 13 a is generated and is fed to the exhaustgas 7 in the exhaust pipe 9.

In FIG. 2, reference numeral 15 denotes a vessel made fromheat-resisting and insulating material such as polyethylene fluoride(e.g., Teflon (registered trademark)). Arranged centrally in the vessel15 is an electrode 16 with its lower end extending adjacent to a bottomof the vessel 15 and with its upper end projected out of and fixed tothe vessel 15. The vessel 15 is encased by a casing 17 made fromelectro-conductive material such as iron, the casing 17 being connectedto an earth 18.

In the embodiment shown in FIG. 2, arranged in the vessel 15 is wirework19 made of stainless steel into which charged are dielectric pellets 19a which in turn may be made from material with high dielectric constantsuch as titania, barium titanate or alumina. The wirework 19 isconnected to the earth 18.

Inserted into and opened in the vessel 15 adjacent to the bottom thereofis a lower end of a urea water feed pipe 20 which serves to feed ureawater 23 a in a urea water tank 23 arranged above the vessel 15 into thevessel 15 via a urea water feed valve 21.

The electrode 16 is connected with power wire 25 which in turn isconnected to a power source 24 such as battery. The power wire 25 isprovided with a controller 26 for control of voltage, driving pulse andthe like. Thus, energization of the electrode 16 by the power source 24generates plasma between the electrode 16 and casing 17, such plasmaacting on the urea water 23 a for decomposition into ammonia and carbondioxide.

Opened to space 27 in the vessel 15 and above a liquid level of the ureawater 23 a is an ammonia feed pipe 28 which is connected via a pump 29and an ammonia feed valve 30 to the spray nozzle 14. Thus, in thisembodiment, the ammonia gas 13 a generated in the space 27 of the vessel15 is taken out through the ammonia feed pipe 28 and fed to the spraynozzle 14. In this connection, if the space 27 is low in volume, theammonia gas 13 a may have difficulty to stably feed; therefore, as shownin FIG. 1, the ammonia feed pipe 28 is preferably provided with anammonia gas storage tank 31 for temporary storage of the ammonia gas 13a generated in the space 27.

In FIGS. 1 and 2, reference numeral 32 denotes a controller into whichinputted is a liquid level signal 34 from a liquid-level meter 33arranged in the vessel 15 for detection of a liquid level. Thus,depending upon the liquid level signal 34 from the meter 33, thecontroller 32 outputs a urea water feed command 35 to control an openingdegree of the urea water feed valve 21 so as to keep constant an amountof urea water 23 a in the vessel 15.

The controller 32 outputs an electricity control command 36 to controlthe controller 26 such that the electricity fed to the electrode 16 haspredetermined voltage and drive pulse.

Further inputted to the controller 32 is a detected pH value 38 from apH meter 37 which detects pH of the urea water 23 a in the vessel 15 (pHadjacent to the liquid level of the urea water 23 a). Thus, dependingupon the detected pH value 38 from the pH meter 37, the controller 32outputs ammonia feed command 39 to control an opening degree of theammonia feed valve 30 to control the flow rate of the ammonia gas 13 afed to the spray nozzle 14. More specifically, the controller 32 and anengine control computer (ECU: Electronic Control Unit) (not shown)exchange data such as revolution speed and load of the engine 1,detected temperatures of inlet and outlet temperature sensors 42 a and42 b for the selective reduction catalyst 10 and intake air amount; onthe basis of a current operational status judged from such data, anamount of NO_(x) generated is presumed. An amount of the ammonia gas 13a to match the presumed generation amount of NO_(x) is calculated sothat the required amount of ammonia gas 13 a is added to the exhaust gas7.

FIG. 3 shows an embodiment with a plurality of such ammonia generators12. Inner space of a casing 17 is partitioned in the form of lattice byelectrically conductive material, each partitioned space of the casing17 receiving the module-structured ammonia generators 12 eachconstituted by the vessel 15. With the plural ammonia generators 12being arranged as shown in FIG. 3, the urea water feed pipe 20 isbranched into a plurality of branch pipes 20 a. Through a urea waterfeed valve 21 in each of the branch pipes 20 a, urea water 23 a is fedto each of the vessels 15. To the respective electrodes 16 of thevessels 15, energization is effected by the power wire 25 in the samecondition. The ammonia gas 13 a generated in the respective vessels 15is taken out together by the single ammonia feed pipe 28.

Next, mode of operation of the above embodiments will be described.

As shown in FIG. 2, with a predetermined amount of urea water 23 a beingfed to the vessel 15 in the ammonia generator 12, electricity is fedfrom the power source 24 (battery) for control of voltage, driving pulseand the like by the controller 26 to a predetermined condition. Then,plasma is generated between the electrode 16 and casing 17 and, by theaction of the plasma generated, the urea water 23 a is decomposed asshown by

(NH₂)₂CO+H₂O→2NH₃+CO₂

into ammonia and carbon dioxide. As a result, the urea water 23 a in thevessel 15 is changed into ammonia water while the ammonia gas 13 a isgenerated in the upper space 27 of the vessel 15, the ammonia gas 13 acontaining carbon dioxide and evaporated water.

As mentioned above, in the ammonia generator 12, the plasma acts on theurea water 23 a for decomposition into ammonia gas 13 a, so that theammonia gas 13 a can be generated easily and quickly.

Further, in this connection, when the dielectric pellets 19 a made ofmaterial with high dielectric constant such as titania, barium titanateor alumina are charged in the urea water 23 a in the vessel 15, plasmais generated on respective surfaces of the pellets 19 a, whichsubstantially enhance decomposition reaction of the urea water 23 a,resulting in effective generation of the ammonia gas 13 a. With theplural ammonia generators 12 being arranged as shown in FIG. 3, theammonia gas 13 a can be generated concurrently in large quantity forsupply.

In the above, by driving the pump 29, the ammonia gas 13 a generated inthe vessel 15 is taken out through the ammonia feed pipe 28 and isinjected by the spray nozzle 14 upstream of the selective reductioncatalyst 10 to be added to the exhaust gas 7 in the exhaust pipe 9.

Then, the controller 32 and the engine control computer (not shown)exchange data such as revolution speed and load of the engine 1,detected temperatures by the inlet and outlet temperature sensors 42 aand 42 b for the selective reduction catalyst 10 and intake air amountto thereby detect the current operational status, so that a generationamount of NO_(x) is presumed on the basis of the detected operationalstatus. An amount of the ammonia gas 13 a to match the presumedgeneration amount of NO_(x) is calculated and the ammonia feed valve 30is controlled by the ammonia feed command 39 so as to feed the requiredamount of ammonia gas 13 a. Since the detected pH value 38 from the pHmeter 37 in the vessel 15 is inputted into the controller 32, thecontroller 32 can calculate ammonia concentration depending upon thedetected pH value 38 of the pH meter 37 to compensate, on the basis ofsuch ammonia concentration, the ammonia feed command 39 for control ofthe opening degree of the ammonia feed valve 30.

Since the liquid level in the vessel 15 is gradually lowered due to thefact that the ammonia gas 13 a decomposed from the urea water 23 a istaken out as mentioned above and due to evaporation of the water, theurea water feed valve 21 is controlled by the controller 32 on the basisof the liquid level signal 34 from the liquid level meter 33 so as tofeed, to the vessel 15, the urea water 23 a adjusted to a predeterminedconcentration in the tank 23, whereby the amount of the urea water 23 ain the vessel 15 is kept constant.

According to the above embodiment, irrespective of the temperature ofthe exhaust gas 7, the ammonia generator 12 generates the ammonia gas 13a by the action of the plasma, the ammonia gas 13 a being injected intoand fed to the exhaust gas 7 in the exhaust pipe 9. As a result, incomparison with the conventional feed of urea water, a required amountof ammonia can be surely added to the exhaust gas 7 even if thetemperature of the exhaust gas 7 is low; thus, even in a vehicle withtravel pattern of continuing the operational status with low exhausttemperature, a sufficient NO_(x) reduction effect can be exhibited evenat exhaust temperature lower than that conventionally required therefor.Since the ammonia gas 13 a causes no problem of lowering the exhausttemperature upon addition to the exhaust gas 7, NO_(x) reduction effectcan be further highly maintained in the operational status with lowexhaust temperature.

In fact, according to experimental results effected by the inventors asshown in the graph of FIG. 7 where comparison was made between a case Xof the above-mentioned embodiment of the invention and a conventionalcase Y of urea water being added to exhaust gas as it is, it has beenactually ascertained that high NO_(x) reduction ratio can be obtainedwith temperature (the inlet exhaust temperature of the selectivereduction catalyst 10 being about 140° C. or so) lower in the case X ofthe inventive embodiment than in the conventional case Y.

Since the ammonia gas 13 a is generated through the action of the plasmaon the urea water 23 a, the generation of the ammonia gas 13 a can beeasily and rapidly adjusted. Since the generated ammonia gas 13 a isadded to the exhaust gas 7, the amount of ammonia gas 13 a to be fed tothe exhaust gas 7 can be controlled with high response.

FIG. 4 shows an embodiment where ammonia water 13 b generated throughaction of plasma on the urea water 23 a in the vessel 15 in the ammoniagenerator 12 of FIG. 2 is injected upstream of the selective reductioncatalyst 10. In this embodiment, the ammonia feed pipe 28 is opened inthe liquid adjacent to the liquid level within the vessel 15. The ureawater 23 a fed by the urea water feed pipe 20 to a position adjacent tothe bottom of the vessel 15 flows upward while decomposed into ammoniaand carbon dioxide through the plasma formed between the electrode 16and casing 17. The ammonia generated by decomposition dissolves in waterso that ammonia water 13 b exists in and especially at the upper part ofthe liquid in the vessel 15. Thus, such ammonia water 13 b is injectedby the spray nozzle 14 through the pump 29 and ammonia feed valve 30into the exhaust pipe 9 upstream of the selective reduction catalyst 10for mixture with the exhaust gas 7.

When the ammonia water 13 b generated in the ammonia generator 12 isadded in this manner to the exhaust gas 7, ammonia in the ammonia water13 b is reacted with NO_(x) and NO_(x) reduction effect can be obtainedjust like the above. Even in a vehicle with travel pattern of continuingoperational status with low exhaust temperature for a long time, asufficient NO_(x) reduction effect can be obtained even at exhausttemperature lower than that conventionally required therefor. Morespecifically, when the ammonia water 13 b is added to the exhaust gas 7,subtle heat may be taken upon evaporation of the water; however,endotherm required for evaporation of the water is lower than heatrequired in a conventional pyrolytical decomposition of the urea water23 into ammonia and carbon dioxide in utilization of heat of the exhaustgas 7. Thus, lowering in temperature of the exhaust gas 7 is subtle;therefore, according to the invention, also in a case where the ammoniawater 13 b is fed to the exhaust gas 7, the NO_(x) reduction effect canbe highly maintained even in an operational status with low exhausttemperature.

FIG. 5 is a schematic diagram showing an embodiment to take out ammoniagas 13 a from an ammonia generator 12 which is structurally differentfrom that of FIG. 2. This ammonia generator 12 is provided with alaterally elongated vessel 40 made from heat-resisting and insulatingmaterial for holding urea water 23 a. In the above within the vessel 40,a plurality of electrodes 16 are arranged in laterally spaced-apartrelationship and are spaced at their lower ends to the liquid level ofthe urea water 23 a by a predetermined distance. The respectiveelectrodes 16 are connected to power wire 25 which in turn is connectedvia a controller 26 to a power source 24 such as battery. Arranged on abottom of the vessel 40 is an electrode plate 41 which is made fromconductive material and is connected to the earth 18. The vessel 40 isfurther provided with a urea water feed pipe 20 similar to that in theabove-mentioned embodiment.

Further arranged in space 27 within the vessel 40 above the liquid levelof the urea water 23 a is an ammonia feed pipe 28 similar to that inFIG. 2 embodiment. The ammonia feed pipe 28 is connected to the spraynozzle 14 via a pump 29 and the ammonia feed valve 30 of FIG. 2.

In FIG. 5 embodiment, only the urea water 23 a is fed in the vessel 40;however, alternatively, just like FIGS. 2 and 4, dielectric pellets 19made from material with high dielectric constant such as titania, bariumtitanate or alumina may be charged in the vessel 40 in addition to theurea water 23 a.

In the FIG. 5 embodiment, through plasma generated by energization ofthe electrodes 16 and electrode plate 41, the urea water 23 a isdecomposed into ammonia and carbon dioxide, ammonia gas 13 a beinggenerated in the upper space 27 of the vessel 15.

Thus, in the ammonia generator 12 of FIG. 5, irrespective of temperatureof the exhaust gas 7, the ammonia gas 13 a can be generated by theaction of the plasma. When added to the exhaust gas 7, the ammonia gas13 a is reacted with NO_(x) to reduce NO_(x); thus, even in a vehiclewith travel pattern of continuing operational status with low exhausttemperature for a long time, a satisfactory NO_(x) reduction effect canbe obtained with exhaust temperature lower than that conventionallyrequired therefor.

FIG. 6 is a diagram showing an embodiment to take out ammonia water 13 bfrom the ammonia generator of FIG. 5. In this embodiment, in order toinject the ammonia water 13 b generated by decomposition of the ureawater 23 a in the vessel 40 through plasma into an exhaust pipe 9upstream of selective reduction catalyst 10, an ammonia feed pipe 28 isopened in the ammonia water 13 b within the vessel 40. Morespecifically, urea water 23 a fed to a position adjacent to the bottomof the vessel 40 by a urea water feed pipe 20 flows upward whilegradually decomposed into ammonia and carbon dioxide by the plasmaformed between electrodes 16 and an electrode plate 41, so that ammoniawater 13 b exits above within the vessel 40. Thus, such ammonia water 13b adjacent to the liquid level is injected upstream of the selectivereduction catalyst 10 via a pump 29 and the ammonia feed valve 30 ofFIG. 2.

Since the ammonia water 13 b generated in the ammonia generator 12 isadded in this manner to the exhaust gas 7, ammonia in the ammonia water13 b is reacted with NO_(x) to obtain NO_(x) reduction effect just likethe above. Thus, even in a vehicle with travel pattern of continuingoperational status with low exhaust temperature for a long time, asatisfactory NO_(x) reduction effect can be obtained with exhausttemperature lower than that required conventionally therefor. Morespecifically, when the ammonia water 13 b is added to the exhaust gas 7,subtle heat may be taken upon evaporation of the water; however,lowering in temperature of the exhaust gas 7 is subtle in comparisonwith a conventional pyrolytical decomposition of the urea water 23 ainto ammonia and carbon dioxide in utilization of heat of the exhaustgas 7. Thus, also in a case of feeding the ammonia water 13 b, theNO_(x) reduction effect can be highly maintained even in an operationalstatus with low exhaust temperature.

It is to be understood that an exhaust emission control device of theinvention is not limited to the above embodiments and that variouschanges and modifications may be made without departing from the scopeof the invention.

INDUSTRIAL APPLICABILITY

An exhaust emission control device of the invention can be effectivelyutilized in effectively generating ammonia from urea water and inenhancing controllability in ammonia addition for obtaining asatisfactory NO_(x) reduction effect even in a vehicle with travelpattern of continuing operational status with exhaust temperature lowerthan that required conventionally therefor.

1. An exhaust emission control device with a selective reductioncatalyst incorporated in an exhaust pipe, ammonia being added upstreamof the catalyst so as to reduce and purify NO_(x), said exhaust emissioncontrol device comprising an ammonia generator with a vessel for holdingurea water and with an electrode for generation of ammonia throughaction of plasma on the urea water in the vessel, the ammonia generatedin the ammonia generator being fed upstream of the catalyst.
 2. Anexhaust emission control device as claimed in claim 1, whereindielectric pellets are charged in the urea water in the vessel.
 3. Anexhaust emission control device as claimed in claim 1, wherein ammoniagas is taken out from the ammonia generator.
 4. An exhaust emissioncontrol device as claimed in claim 2, wherein ammonia gas is taken outfrom the ammonia generator.
 5. An exhaust emission control device asclaimed in claim 1, wherein ammonia water is taken out from the ammoniagenerator.
 6. An exhaust emission control device as claimed in claim 2,wherein ammonia water is taken out from the ammonia generator.
 7. Anexhaust emission control device as claimed in any one of claims 1-6further comprising a pH meter for detecting concentration of ammoniataken out from the vessel and a controller for outputting a command onan amount of ammonia to be fed upstream of the catalyst on the basis ofthe detected value from the pH meter.